F/F ???. Can be a real problem - especially if it has problems loading a page. I tend to have a few (8-10) tabs open, and I've found even with the latest nvidia (proprietary) driver on my core2 duo laptop, X will quite happily drive one core mad. No flash, no nuthin.
Kill F/F, and the problem goes away. It's a pig - I only use it because of the plugins.